Q128C specs selection help

1N4001

100 W
Joined
Nov 28, 2017
Messages
108
Greetings,

I am planning a custom commuter build. I have a couple of criteria that offer a challenge:

- Rear hub motor with cassette
- Cruising speed 50kph, max speed 60kph on flats. Manageable with aero road handlebars and 1300W peak power (1050W motor, 250W legs).
- Decent torque and acceleration
- Lightweight

60kph in a 29" wheel is about 450rpm, so I will require roughly 450 * 1.2 = 540 unloaded rpm. This is my target. Please correct me if I am wrong!

I've settled on a Q128C. They're very lightweight and said to handle 1000W without much trouble. There are four versions of it:

36V 201rpm = 5.58rpm/V
36V 328rpm = 9.11rpm/V
48V 201rpm = 4.19rpm/V
48V 328rpm = 6.83rpm/V

36V/328rpm would be the obvious choice, it'll only need 60V to reach the desired rpm. However I'm doubtful that it'll have much torque due to being wound so fast.

The next fastest is 48V/328rpm. It'll require a whopping 80V for the desired rpm. Which means 22s battery and a very limited controller selection. On the other hand, more torque. And trouble managing phase current??

The other two motor versions I will disregard.


What do you think? Which variant is preferable? Are there any other glaring flaws in my plan?

Thanks a lot!
 
You'll need to elaborate on your choice of frame, riding style, terrain/topography before anyone can properly advise.

The Q128c motor is a nice motor, but it isn't designed to handle these power outputs and speeds in any situation; personally, I wouldn't put anything over 800W at 52V through a Q128c. I'm sure others will comment, but if you are looking for a geared motor that can handle 1500W, the Mac from EM3EV would be a better choice. The Mac 8T at 52V in 29er would get just over 50kph, whereas the 6T would get just over 60 kph. Putting anything in a large wheel will result in lousy torque and put a lot of stress on the motor. Better to go higher voltage in a smaller diameter wheel if possible.

Some get away with small cutes in 700c or 29" wheels, but they are athletic and use the motor solely to augment leg power. These builds will typically achieve 45-50 kph max; at 60kph, wind resistance is increasing exponentially and you'd really need a stronger motor - thereby losing the bike-like properties of your build.

Always compromises...
 
Here is a really long thread where several of us debated the practically of trying to use a Q128 for 30 +mph build;

https://endless-sphere.com/forums/viewtopic.php?f=2&t=90904&hilit=Q128
 
Thank you for your replies.

molybdenum said:
You'll need to elaborate on your choice of frame
Planned is a light yet sturdy Titanium frame. No carbon.

riding style, terrain/topography
As a commuter, mostly straight ahead on flat roads at cruising speed, combined with stop&go at traffic lights, intersections, etc. Occasionally gravel touring aswell, some minimal climbing capability (5%) would be nice too. Both obviously not at full speed.

but if you are looking for a geared motor that can handle 1500W, the Mac from EM3EV would be a better choice. The Mac 8T at 52V in 29er would get just over 50kph, whereas the 6T would get just over 60 kph.
I did consider a MAC, but it's also a lot heavier, larger and limits my controller selection due to its pole count. I like the Q128c for being compact, lightweight and quiet.

The Q128c motor is a nice motor, but it isn't designed to handle these power outputs and speeds in any situation; personally, I wouldn't put anything over 800W at 52V through a Q128c.
With a cruising speed of 50kph, I'll need far less power, so 800W should suit me fine. Subtract another 200W coming from leg power, so realistically we are looking at 600W continuous, and 1100W peak.

Putting anything in a large wheel will result in lousy torque and put a lot of stress on the motor. Better to go higher voltage in a smaller diameter wheel if possible.
Thanks, that's exactly the sort of information I'm looking for. Smaller wheels are not an option for me, but higher voltage should be doable. But won't a very high voltage make it harder to manage phase currents?

at 60kph, wind resistance is increasing exponentially and you'd really need a stronger motor - thereby losing the bike-like properties of your build.
Always compromises...
Too true. I am trying to create a bike-like build, while still allowing high speeds. Maybe if I only eat half of my cake, I can keep half of it too... :)

motomech said:
Here is a really long thread where several of us debated the practically of trying to use a Q128 for 30 +mph build;

https://endless-sphere.com/forums/viewtopic.php?f=2&t=90904&hilit=Q128
I've seen it (even replied in it). It's very informative and one of the reasons I settled on a Q128c. I don't need as much battery capacity, so I think I can get away with less weight and slightly more speed still.
 
Any other comments, opinions or information? Is anything I mention incorrect or in need of further consideration?
 
1N4001 said:
Any other comments, opinions or information? Is anything I mention incorrect or in need of further consideration?

Depending upon who's post you read and when, there is a lot of speculation that the "201" and "328" (and for some motors "260") are not direct readings of the maximum unloaded kV for the actual motors, but rather historic designations relating to an unspecified series of motors that did have these Kvs, and the designations stuck, whilst the kVs changed over time as windings and gear ratios varied.

eg. the various differing designations and "actual" kVs listed here.

A search with "site:endless-sphere.com (Q128 OR Q128C OR Q128H OR Q128CST) (201 OR 260 OR 328)" find 484 or 207 hits depending on whether you believe the number displayed on the first 4 pages of results or the latter shown on the 5 and subsequent ones :(

(Omit the terms you aren't interested in; but understand that the Q128H is the more capable (faster/more torque) motor if you can live with 7-speed thread-on.)
 
I'm not a fan of the Q128H, as I want to get away from these awful freewheels and use a freehub.

You're right about the speeds. here it was mentioned that 36V/328rpm motor is 10.8rpm/V (unloaded) instead of 9.11rpm/V. And here it is theorized that the 48V/328rpm and 36V/201rpm motors are the same, being 36V/260rpm (7.2rpm/V) instead. I can only assume they mean loaded speed.

Getting a straight answer out of BMSBattery is an excersize in futility, so I won't bother.


I want to focus on that 48V/328rpm|36V/260rpm variant. If the thread on pedelecs.co.uk is to be trusted, and my assumption that they are talking about loaded speeds is correct, a 67V battery should suit me fine, giving me 64kph (assuming 7.2rpm/V).

Any objections?
 
1N4001 said:
I want to focus on that 48V/328rpm|36V/260rpm variant. If the thread on pedelecs.co.uk is to be trusted, and my assumption that they are talking about loaded speeds is correct, a 67V battery should suit me fine, giving me 64kph (assuming 7.2rpm/V).

Any objections?

No objections. I'm as much in the dark as you are.

But, I have seen comments that going above ~52V is unlikely to get you any more speed, and you risk melting the phase wires without you rewire with a heavier gauge and there's limited room in the axle.

Good luck. Please report back your results.
 
Many have found that at 48V, the Q128c is near saturation and even going to 52V does not provide returns proportional to the voltage increase. Moreover, a Q128c 328RPM@48V at 800W and with another 150W leg power in a lightweight bike rider tucked will achieve 30 mph. An underamped motor will simply not go any faster at higher voltage irrespective of the motor type, as it is the amps that overcome resistance and push the motor closer to the unloaded speed. 60+ volts is too high imo, and at 800W, amps are limiting; you will achieve nothing over lower voltages. You should run the Q128c at 52V max and optimize bike, tires, rider position etc for speed.
 
Yes, I am fully aware that voltage alone will not make a motor faster. I have to balance two factors to get to my top speed:

- Motor current, which will have to be carefully monitored and limited by the controller.
- Battery voltage, which ensures that I am still able to drive this current into the motor even at high speeds.

If I can drive the motor with enough amps to keep it in the 1000W range, that should be enough power to get up to 60kph.

Current is everything, it's proportional to magnetic flux, torque and heating. There's no direct relationship between voltage and current, as it is the controller's job to manage that. The battery voltage just needs to stay above the voltage generated by the motor.

I really don't see why a high voltage could bring the motor into saturation. Magnetic flux is soley proportional to current. Or am I missing something here?


molybdenum said:
optimize bike, tires, rider position etc for speed.
This is already being done, regardless of motor considerations :)

Buk___ said:
Good luck. Please report back your results.
Thanks, and will do ;)
 
1N4001 said:
I really don't see why a high voltage could bring the motor into saturation. Magnetic flux is soley proportional to current. Or am I missing something here?

Cores saturate. With the small (18/20mm wide) and narrow teeth, once the teeth are fully saturated, even if you can get more volts and amps into the windings without melting them, you won't increase the flux, so no more torque or speed. You'll just generate more heat.

(The H variant has (apparently) a wider stator and magnets, so you can push it a little further than the others.)
 
Correct. But the voltage won't have an influence on that, only the current determines flux. You are of course both correct, that this will limit the motor's maximum power output. But I'm guessing it'll still be able to reach 1000W before saturation.
 
1N4001 said:
Correct. But the voltage won't have an influence on that, only the current determines flux.

Unless you can increase the flux, adding more volts won't increase your speed; so will just generate heat.
 
Buk___ said:
Unless you can increase the flux, adding more volts won't increase your speed; so will just generate heat.
that's what I said :D

Here's a simulation of a motor that I think comes closest to the Q128. Note that I am limiting phase current to 20A throughout.
xWP02c8.png
 
1N4001 said:
Buk___ said:
Unless you can increase the flux, adding more volts won't increase your speed; so will just generate heat.
that's what I said :D

Here's a simulation of a motor that I think comes closest to the Q128. Note that I am limiting phase current to 20A throughout.
xWP02c8.png

There are a few things wrong with that simulation.

1) "I think comes closest" is a poor statement. If that other motor has a larger diameter and/or wider stator; or fewer slots allowing more core bulk per winding; or bigger magnets; or better (thinner) laminates; or fewer, thicker turns of copper; or any of a number of other variations, it will perform differently in the real world.

2) It is not at all clear (to me at least), how much of that the simulator takes into consideration.

3) In the graph you posted, at the speed indicated, the load (black) curve is far in excess of the torque (blue) curve, and even well in excess of the (red) power curve.

My guess (and it is little more, I can make no sense of that simulation), is that your real world speed would be somewhere between the load/torque crossover at ~41kph and the load/power crossover at ~51kph.

I know why the simulator positions the speed line there -- its all to do with that acceleration: -0.00kph bottom right -- but how it arrives at that conclusion, ie. the math behind it, leaves me perplexed.

I think anything beyond 50 kph is just wishful thinking; but I'd dead chuffed for you to prove me wrong! It would be a very pleasant thing to discover when I get around to upping the battery for my Q128H, from 36V to 48V, that it goes substantially faster than I am expecting.
 
Buk___ said:
1) "I think comes closest" is a poor statement. If that other motor has a larger diameter and/or wider stator; or fewer slots allowing more core bulk per winding; or bigger magnets; or better (thinner) laminates; or fewer, thicker turns of copper; or any of a number of other variations, it will perform differently in the real world.
I did simulate a few other motors too, including the Q128's smaller brother Q100, which manages 54kph with the same settings (theoretically, not taking into account melting windings)

3) In the graph you posted, at the speed indicated, the load (black) curve is far in excess of the torque (blue) curve, and even well in excess of the (red) power curve.
Yes, that is the result of the additional human power.

My guess (and it is little more, I can make no sense of that simulation), is that your real world speed would be somewhere between the load/torque crossover at ~41kph and the load/power crossover at ~51kph.

I know why the simulator positions the speed line there -- its all to do with that acceleration: -0.00kph bottom right -- but how it arrives at that conclusion, ie. the math behind it, leaves me perplexed.
The load curve (black) represents the power needed for a certain speed. It takes into account losses due to wind and rolling resistance. Max speed is achieved when motor power (red) equals load. I gain a few more kph due to added human power, which the graph handles by positioning the selector 250W further on the load line.

I am confident that if I can avoid core saturation or meltdown @20A, I can reach at least 55kph.
 
1N4001 said:
I gain a few more kph due to added human power, which the graph handles by positioning the selector 250W further on the load line.

I am confident that if I can avoid core saturation or meltdown @20A, I can reach at least 55kph.

If a 28" wheel has a rolling radius of 2.2m, that's 416.67 wheel rpm. Using (say?) a 44/12 gear, you're going to add 250w sustained at a cadence of 113?

Best of luck! Let us know how you get on :)
 
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